docs: def Lewis(D=None, alpha=None, Cp=None, k=None, rho=None):
      r'''Calculates Lewis number or `Le` for a fluid with the given parameters.
  
      .. math::
          Le = \frac{k}{\rho C_p D} = \frac{\alpha}{D}
  
      Inputs can be either of the following sets:
  
      * Diffusivity and Thermal diffusivity
      * Diffusivity, heat capacity, thermal conductivity, and density
  
      Parameters
      ----------
      D : float
          Diffusivity of a species, [m^2/s]
      alpha : float, optional
          Thermal diffusivity, [m^2/s]
      Cp : float, optional
          Heat capacity, [J/kg/K]
      k : float, optional
          Thermal conductivity, [W/m/K]
      rho : float, optional
          Density, [kg/m^3]
  
      Returns
      -------
      Le : float
          Lewis number []
  
      Notes
      -----
      .. math::
          Le=\frac{\text{Thermal diffusivity}}{\text{Mass diffusivity}} =
          \frac{Sc}{Pr}
  
      An error is raised if none of the required input sets are provided.
  
      Examples
      --------
      >>> Lewis(D=22.6E-6, alpha=19.1E-6)
      0.8451327433628318
      >>> Lewis(D=22.6E-6, rho=800., k=.2, Cp=2200)
      0.00502815768302494
  
      References
      ----------
      .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
         Eighth Edition. McGraw-Hill Professional, 2007.
      .. [2] Cengel, Yunus, and John Cimbala. Fluid Mechanics: Fundamentals and
         Applications. Boston: McGraw Hill Higher Education, 2006.
      .. [3] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
         Berlin; New York:: Springer, 2010.
      '''
      if k and Cp and rho:
          alpha = k/(rho*Cp)
      elif alpha:
          pass
      else:
          raise Exception('Insufficient information provided for Le calculation')
      return alpha/D